My suggestion would be to have the vet do a necropsy to determine cause of death (like a human autopsy) to keep this from being a guessing game that causes hard feelings.
It is possible the dog got a burn on its head or was slipped or helped himself to something a dog shouldn't eat if it was around during the party. An inflamed pancreas from some fatty treats, a bone that caused an intestinal tear and infection, chocolate poisoning, licking up any spilled fuel oil, being out in the heat too long - so many things could cause the symptoms you saw. The spot on the head might have been related or not to the entire illness.
I think for the sake of your own knowledge and family peace a necropsy would be your best bet.
And as you lost a dear pet you may find www.petloss.com and the rainbow bridge story may bring you some small comfort.
Hope this helps you!
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